158 Manual of natural history. 



terminal ; hinge usually edentulate, with a 

 transverse septum. Fluviatile. 



3. Family. — Pearl-Oysters (Aviculidae). Mantle 



freely open, margins cirrhated ; foot small, 

 cylindrical, with a byssal groove ; shell foli- 

 ated, irregular, pearly within, right valve 

 with a notch for the byssus ; hinge-margin 

 straight ; ligament marginal, simple or inter- 

 rupted. 



4. Family. — Pinnas (Pinnidse). Mouth with folia- 



ceous lips ; no separate posterior opening ; 

 anal siphon with a long ligulate valve ; gills 

 foliaceous ; shell wedge-shaped, gaping at 

 ventral margin, pointed at dorsal ; hinge 

 lateral, without teeth ; ligament linear, al- 

 most internal. 



5. Family.— Clams (Tridacnidse). Mantle closed, 



except for the branchial and anal orifices, and 

 the aperture for the thick, cylindrical, byssi- 

 ferous foot ; shell regular, transverse, truncate * 

 hinge with two compressed teeth ; ligament 

 external ; muscular scars united, irregular. 



II. SUB-CLASS. — Unimuscular-Bivalves 

 (Monomyaria). 



Animal with a single adductor muscle for closing 

 the valves. 



I. OBDEK.— Micropods (Micropoda). 



Mantle-lobes entirely free ; siphons none ; foot 

 rudimentary. 



